You'd think it'd be hard finding things to say about Terry Pratchett and his Discworld creations, given that I Shall Wear Midnight is his 39th novel set in the parallel world and the fourth in a young adult series starring teenage witch Tiffany Aching.

And yet the modern master of comic fantasy always leaves you with things to ponder long after his characters have sailed/flown/limped into the sunset.

I Shall Wear Midnight picks up Tiffany's story as she settles - or not - into life as "town witch" on The Chalk, taking care of the things people generally don't like to think about.

There, with the assistance of the spectacularly argumentative, kilt-wearing, wee but hardy Nac Mac Feegle, she tends to the needs of her village, always riding a knife-edge between being useful and being an object of suspicion who meddles in unmentionables.

But Tiffany's skills as a witch have caught the attention of the Cunning Man (surely one of Pratchett's spookiest villains), a no-eyed spectre who menaces our heroine as she goes about the business of seeing her village through a change in baron.

Fans of Pratchett will find much here to love, not least the fact that Tiffany is a younger, livelier, possibly humbler version of one of his most-loved creations - Granny Weatherwax. She is wise beyond her years as a result of facing life's uglier side more often than your average 15-year-old.

Pratchett's stories have become gradually darker over the years. The temptation is to attribute that to the author's struggles with Alzheimer's. But it seems more likely that familiarity with his world and his ever-increasing success, means he can bring the grimmer elements that have always been a part of his stories to the forefront.

The discrepancy in Midnight is that while it is in parts among Pratchett's darkest tales, it is, at least nominally, a book for younger readers. While anyone who's ever met a "younger reader" would never suggest they only want to read books full of sweetness and light (and the occasional heart-throbby vampire), there's still a contradiction here. Pockets of the story are very dark, dealing with domestic violence and mob mentality, and yet the story arc is noticeably superficial.

The Cunning Man is a brilliantly disturbing villain, yet he feels underused and is too easily dealt with in a slightly rushed conclusion. It's as though the author couldn't quite decide what to do with so much menace.

The upside of a simplistic plot, though, is that the spotlight shines squarely on his characters and Pratchett is always at his best portraying those who are wry and brutally honest as they dance with the dark. In that regard, Tiffany Aching is one of his best.

As a treat for fans - it doesn't add much to the plot - Pratchett takes Tiffany to his famous city of Ankh Morpork, where she meets and mingles with other fan-favourite characters and there's a satisfying pay-off in Aching's relationship with other Discworld witches.

At that point, if you're already a fan you just don't care too much about plot resolution; you're simply a big, contented geek ready to a don a costume for a convention.

So I Shall Wear Midnight is not a perfect Pratchett - but it is fierce, dark, intelligent and still streets ahead of anybody else in his hemisphere.